Explanation of Pickups?

Hey im kinda a n00by but kinda not on bass, like i know alot of stuff, then agian i dont know alot of stuff, like the only thing i know about pickups is j and p, for jazz and precision, and active and passive and stuff, but can someone please try to explain like the purpose of pickups, which brands are good, how a pickup works, which pickups are good for which Genre's, etc. etc.? thaaaanksss

Pickups pick up the sound of the strings. Think of them as microphones. Usually these are "magnetic pickups." The way they work is each pickup has one or more magnets. These are under the strings, usually as "pole pieces" or a blade. The pole pieces might be separate magnets, as on Fender PUs, or be metal slugs with a magnet underneath.

Around the magnets is a coil of very thin magnet wire. This wire is about as thin as a hair. For high impedance "passive" pickups, there are usually many thousand turns of wire. So for a pickup reading 12k (12,000 ohms) you might have about 12,000 turns of wire.

When the metal strings move through the magnetic field created by the pickups, an electrical current is formed and flows through the coil and out to the volume and tone controls. This signal is not very loud, so it goes to your amplifier, first to the preamp and then the power amp, and (hopfully) you hear it as musical notes.

Active pickups differ in that often the coils in the pickup are "low impedance" ... that is they are wound with much fewer turns. If a high impedance PU has 12,000 turns, a low impedance PU might have between 200 and 2,000. By doing this you get a much cleaner and more natural sound, with less coloration. Although we general accept the coloration as normal. Because low impedance PUs use far less turns of wire, their output is far less than the high impedance PUs, and they need an additional preamp to bring them up to either to instrument or line level.

Usually active basses also have active tone controls.

The traditional Fender shapes such as the P and J are still used, as well as more modern "soap bar" shapes.

As far as which pickup to use for what? That's all a matter of opinion. I look for a clean and clear tone with no noise and hum. With that you can get any sound your bass and amp are capable of.